Electroconductive polymeric materials such as polyacetylene, poly-p-phenylene, polypyrrole, polythiophene, and polyaniline are light weight and flexible a compared with metal materials. Such conductive polymeric materials are expected to make it possible to fabricate flexible electrochemical cells of lighter weight. Research effort has been focused on the use of conductive polymeric materials as the cell electrode. The conductive polymeric materials are generally prepared by either an electrolytic polymerization or a chemical process of oxidative polymerization in the presence of an oxidizing agent such as iron chloride. Those materials prepared by electrolytic polymerizaton are known to show superior performance as electrode material compared to those materials prepared by chemical oxidative polymerization because the former is available as a film having a high conductivity. In particular, polymers of aniline and its derivatives are preferred electrode materials. Among others, polyaniline is of greatest interest because it has a remarkably high charge/discharge capacity and is thus most practical as cell electrode material.
A typical prior art process for fabricating an electrochemical cell using a conductive polymeric material like polyaniline as an electrode active material includes the following steps. First, a conductive polymeric material is synthesized and deposited on an electrode as a film by electrolytic polymerization. The film of polymeric material is separated from the polymerization electrode and attached to a collector to form a cell electrode, which is used to assemble a cell. The steps of separating a conductive polymeric film from a polymerization electrode at the end of electrolytic polymerization, attaching the film to a collector to form an electrode unit, and placing the unit in a container are so cumbersome that the overall cell assembly process is complicated and time consuming. When separated film is again joined to the collector, the resulting film-collector assembly has an increased internal resistance due to an increase of resistance at the interface. A cell structure resulting from these steps imposes some limitation on the size of a cell although more compact, thin cells are currently demanded.
The collector is generally interposed between an exterior container and an electrode active material in a cell. An electrode is formed by placing the active material on one surface of the collector as a layer. In general, however, the collector which is formed of metal and carbon materials and the active material which is a conductive polymeric material have different physical properties. Because of this difference, there is the likelihood that the electrode would warp during drying or upon impregnation with electrolyte to render it difficult to assemble a cell, or the collector and the active material would separate from one another to further increase the interfacial resistance.
A secondary cell wherein a composite unit of a collector having a conductive organic polymeric material integrated on one surface thereof is placed in an exterior shell as an electrode has another problem The polymeric material expands by taking in anions or cations from the electrolyte during charging, and contracts by releasing anions or cations during discharging. In other words, the polymeric material undergoes a volume change due to electrochemical doping and de-doping during charging and discharging processes. The secondary cell having such a conductive organic polymeric material as an electrode active material maintains good electrical connection in a charging state because the expansion of the polymeric material forces the collector in firm contact with the exterior shell. As discharge proceeds, the polymeric material contracts to loosen the mechanical contact between the collector and the exterior shell to aggravate the electrical connection between the collector and the exterior shell, drastically increasing the internal resistance of the cell. This phenomenon is frequently observed particularly in coin- and button-type cells.